#include <errno.h>
#include <stdlib.h>
#include <wchar.h>

#include "internal.h"

size_t mbrtowc(wchar_t* restrict wc, const char* restrict src, size_t n, mbstate_t* restrict st) {
  static unsigned internal_state;
  unsigned c;
  const unsigned char* s = (const void*)src;
  const unsigned N = n;
  wchar_t dummy;

  if (!st)
    st = (void*)&internal_state;
  c = *(unsigned*)st;

  if (!s) {
    if (c)
      goto ilseq;
    return 0;
  } else if (!wc)
    wc = &dummy;

  if (!n)
    return -2;
  if (!c) {
    if (*s < 0x80)
      return !!(*wc = *s);
    if (MB_CUR_MAX == 1)
      return (*wc = CODEUNIT(*s)), 1;
    if (*s - SA > SB - SA)
      goto ilseq;
    c = bittab[*s++ - SA];
    n--;
  }

  if (n) {
    if (OOB(c, *s))
      goto ilseq;
  loop:
    c = c << 6 | (*s++ - 0x80);
    n--;
    if (!(c & (1U << 31))) {
      *(unsigned*)st = 0;
      *wc = c;
      return N - n;
    }
    if (n) {
      if (*s - 0x80u >= 0x40)
        goto ilseq;
      goto loop;
    }
  }

  *(unsigned*)st = c;
  return -2;
ilseq:
  *(unsigned*)st = 0;
  errno = EILSEQ;
  return -1;
}